The relation between radiation-induced mutations and chromosome breaks is not understood at the present time; a new hypothesis of the whole animal from only one of the first two cleavage products appears to resolve some of the problems. The research will focus on the study of the production of mosaics in Drosphila melanogaster as a function both of X-ray dose and chemical treatment, and the computer analysis of genetic data on chromosome loss and dominant lethal production in view of this hypothesis.